In switched-mode drivers, such as those used for driving an LED, an output buffer capacitor is repeatedly charged and discharged. In case the driven LED is defective to provide an open circuit, or no LED is connected to the output terminals, the voltage over the output buffer capacitor may become very high, and thus the voltage over the output terminals may become very high, which is potentially dangerous and may cause damage to the electronic components. Known protection circuits comprise a relatively large number of components and are hence relatively costly. For example, numerous transistors, stabilivolt, capacitors, etc are used for sensing the voltage of a secondary transformer winding which is indicative of the load voltage. Additionally, the current open protection circuit starts and stops alternatively and results in burping of the driver; re-start again and again.
US2005/0093488 also discloses an IC-based LED driver. The switching of the power switch MOS 461 is controlled by an LED line driver IC 420. A startup of the power switch is handled by the IC, not by a current from the power supply via the load. Such IC-based solution is high cost.
US2005/0093488 also discloses an IC-based driver. But it is a linear driver, not a switching power driver. A transistor is parallel with the load and is used for as a resistive element to shunting a portion of power from the load. Also, the transistor 108 in this prior art controlled by the IC and is not turned on by a startup current coming from the power supply via the load.